The bomb, which exploded late Wednesday in Ruso district of Narathiwat province, also seriously wounded a fifth soldier and destroyed their jeep, said Police Lieutenant Srithanon Samlee.

A complex insurgency, without clearly stated aims, has plagued Thailand's far south near the border with Malaysia since 2004, claiming thousands of lives, both Buddhist and Muslim, in near-daily bomb or gun attacks.

Deputy Prime Minister Yutthasak Sasiprapa described the latest bombing as a "failure of tactics not strategy," saying troops had been told not to take the same roads back to their barracks each day.

He said that while there had been fewer attacks recently the severity of the incidents had increased.

People in the region complain of a long history of discrimination against ethnic Malay Muslims by authorities in the Buddhist-majority nation, including alleged abuses by the armed forces, which have a heavy presence.

In a separate attack, a 42-year-old Muslim paramilitary ranger was killed by a bomb in neighbouring Pattani province on Wednesday while on patrol on his motorcycle, police said.

Shortly afterwards security forces clashed with six suspected militants, one of whom was killed in the gunfight, according to the authorities.